1. Field of the Invention
The present invention relates to a digital broadcasting receiver, and a method for compensating a color reproduction error therein, in which a color reproduction error of a picture signal is minimized when there is a system clock variation with reference to a Program Clock Reference (PCR).
2. Background of the Related Art
Following the ceaseless effort for putting digital broadcasting technologies into practical use for recent several years, an air borne digital television (DTV) broadcasting wave reaches even to living rooms, and receivers are supplied, partly. Starting from unitary models, there are a variety of models of the digital broadcasting receivers, such as separable set-top model.
Though there may be countless development, such as channel coding systems, various additional services, differences of the DTV broadcasting from the present analog TV broadcasting may be the extended audio channels and the quality of the high definition of video in view of audio/video that are ultimate objects of the television broadcasting stations. Especially, a high definition (HD) class digital program has achieved a resolution of five times high quality compared to the present analog broadcasting program.
A DTV signal receiver set-top provides picture signals of different formats for an HD class component, a Standard Definition (SD) class component, the present analog composite picture signals, and the like.
Of the picture signal formats, the HD class picture signal format mostly supports two kinds of chromaticity systems of a YPbPr component signal in which a synchronization signal (Sync) is included to a luminance signal Y and red, green, and blue (RGB) Hsync/Vsync having a horizontal synchronization signal component and a vertical synchronization signal component split, and two kinds of resolutions of HD1080i and HD720p.
Moreover, the HD class picture signal format has an interchangeability with a display method in which the HD class is down converted to 480 p or 480 i, both of which are of the SD Aclass, and put into practical use presently.
Particularly, it is essential that the DTV receiver has an interchangeability with the present analog TV receiver. Accordingly, all HD, and SD class DTV receivers convert a received digital broadcasting signal into an analog composite picture signal before presenting the digital broadcasting signal.
There is a big difference between receiving the present analog TV broadcasting signal and presenting the signal as an analog signal, and receiving a digital broadcasting signal and presenting the signal as an analog signal.
First of all, the analog picture signal has a form of a composite picture signal as a sync component, a color sub-carrier and the like modulated with a picture signal are demodulated with the picture signal. Therefore, the analog receiver is not required to re-construct the sync component, the sub-carrier, and the like.
However, the DTV receiver can convert a picture format of a received stream into a different format, or frame ratio.
Therefore, the synchronization signal and the like are re-constructed to be in conformity with a format to be presented finally, especially, when the picture to be present is an analog composite picture signal, the sync and the color sub-carriers are re-constructed through a digital NTSC (National Television System Committee) encoder.
Accordingly, variation of the system clock is required for recovery of the PCR from a digitally received picture processing block operative under single system clock. The system clock is varied as much as the variation of the NTSC sync and color sub-carrier in the re-construction.
When a general VCXO (Voltage Controlled Crystal Oscillator) is used, the system clock can be varied up to 100 ppm in an unsteady state, according to which the sub-carrier frequency is also varied up to 100 ppm.
In this case, if the sub-carrier frequency is not compensated, it is liable that a color demodulation error can be occurred at an analog receiver which receives a composite picture signal from a digital broadcasting receiver. In an extreme case, the composite picture signal is out of a range of color demodulation.
Therefore, a digital broadcasting receiver designed to be operative under single system clock is required to compensate the color sub-carrier for a system clock variation.